Although adolescent physical activity participation has clear short and long-term health benefits, nearly half of American youth are not physically active on a regular basis. In order to design successful interventions and policies for enhancing adolescent physical activity it is important to understand the determinants of different adolescent physical activity patterns and how patterns change over time. The proposed study adopts a social contextual, developmental perspective to studying childhood and adolescent physical activity. Within the developmental social context model, the influences of demographic, person, family, peer, school, and neighborhood factors will be assessed over time as will relationships between adolescent physical activity patterns, substance use, diet and depression. The study Will assess 360 children from three cohorts (10, 12, and 14 years), a primary care giver, and a peer "best friend" of the child, annually over four years. Thus, within the proposed cohort-sequential design, we will be able to examine physical activity behavior over eight years of development (ages 10- 17 years). In addition to focusing on individual behavior, the study will also determine the extent of homogeneity in family physical activity patterns and examine the influence of contextual characteristics on changes in family physical activity over time. Given the longitudinal and hierarchical nature of the study, the data will be analyzed using state-of-the-art analytic techniques specifically designed for such data. Specifically, the use of latent growth curve methodology, generalized linear modeling, and hierarchical/multi-level modeling techniques will enable us to examine changes in physical activity patterns during the developmental period of interest, as well as the changing influences of the hypothesized antecedent variables across multiple contextual levels. This longitudinal study of the dynamic interplay of social contexts is likely to lead to a greater understanding and identification of malleable personal and contextual factors that can be targeted for physical activity intervention programs at different stages of adolescent development.